A constant warning time device (often referred to as a crossing predictor or a grade crossing predictor in the U.S., or a level crossing predictor in the U.K.) is an electronic device that is connected to the rails of a railroad track and is configured to detect the presence of an approaching train and determine its speed and distance from a crossing (i.e., a location at which the tracks cross a road, sidewalk or other surface used by moving objects). The constant warning time device will use this information to generate a constant warning time signal for controlling a crossing warning device. A crossing warning device is a device that warns of the approach of a train at a crossing, examples of which include crossing gate arms (e.g., the familiar black and white striped wooden arms often found at highway grade crossings to warn motorists of an approaching train), crossing lights (such as the red flashing lights often found at highway grade crossings in conjunction with the crossing gate arms discussed above), and/or crossing bells or other audio alarm devices. Constant warning time devices are often (but not always) configured to activate the crossing warning device at a fixed time (e.g., 30 seconds) prior to an approaching train arriving at a crossing.
Typical constant warning time devices include a transmitter that transmits a signal over a circuit formed by the track's rails and one or more termination shunts positioned at desired approach distances from the transmitter, a receiver that detects one or more resulting signal characteristics, and a logic circuit such as a microprocessor or hardwired logic that detects the presence of a train and determines its speed and distance from the crossing. The approach distance depends on the maximum allowable speed of a train, the desired warning time, and a safety factor. Preferred embodiments of constant warning time devices generate and transmit a constant current AC signal on said track circuit; constant warning time devices detect a train and determine its distance and speed by measuring impedance changes caused by the train's wheels and axles acting as a shunt across the rails, which effectively shortens the length (and hence lowers the impedance) of the rails in the circuit. Multiple constant warning devices can monitor a given track circuit if each device measures track impedance at a different frequency.
Federal regulations mandate that a constant warning time device be capable of detecting the presence of a train as it approaches a crossing and to activate the crossing warning devices in a timely manner that is suitable for the train speed and its distance from the crossing. In addition, the device must be capable of detecting trains that approach the crossing from both sides of the crossing (e.g., from east to west and from west to east, north to south and south to north, etc.).
In recent years, the North American rail industry has seen an increased number of events in which constant warning time devices have not performed as expected. Although the exact root cause of the events cannot be determined, it appears that the events are based on the rail vehicle (e.g., a train) not presenting a 0.06 ohm shunt between the lead axles of the train and the rail surface. All AREMA (American Railway Engineering and Maintenance-of-Way Association) based equipment and FRA (Federal Railroad Administration) testing is based on those values. The events appear more often for newer, faster and lighter passenger trains, which present a different effective shunt than standard freight trains.
Moreover, there are a number of changes in railroad operations that have led to the potential for a “dirtier” rail than in the past such as e.g., (1) more use of rail lubricants to reduce wheel and rail wear; (2) increased use of dynamic braking for train speed control instead of air brakes, which reduces the amount of time that the brake shoes can contact the wheel treads and scrub off any dirt or contamination collected on the wheel; (3) increased use in the rail passenger fleet of the use of disc braking systems that do not provide a scrubbing action on the wheel tread that contacts the rail; and (4) changes in the compounds used to make up the brake shoes themselves and changes in the metallic structure of the rail itself.
Currently, there is no system that is capable of determining the shunting characteristics of a moving train or other rail vehicle. Being able to determine the correct value of an effective shunt, specifically for certain types of passenger trains, can lead to optimizing the performance of the crossing warning time device when it is presented with a shunt value that does not meet the 0.06 ohm standard. Thus, there is a need and desire for a technique for detecting the dynamic train-to-rail shunting performance of a train as it is moving along the rails of a railroad track.